1. Field of the Invention
This invention relates to an apparatus for scanning tracks on a recording medium with a light spot while effecting tracking control, thereby accomplishing the recording and reproduction of information.
2. Related Background Art
Various media such as a disk-like medium, a card-like medium and a tape-like medium are known as forms of medium on which information is recorded by the use of light and from which the recorded information is read out. These optical information recording media include media capable of recording and reproduction and media capable of reproduction alone. Particularly, an optical card as a recording medium is expected to be extended because of its ease of manufacture, its good portability, its good accessibility, etc. There have been proposed various optical information recording and reproducing apparatuses to which the optical card is directed.
In an optical information recording and reproducing apparatus, recording and reproduction are effected while auto tracking and auto focusing control is always effected. Also, the recording of information onto a recording medium is accomplished by scanning an information track with a light beam modulated in accordance with recording information and stopped down into a minute spot, and information is recorded as an optically detectable information pit row. The reproduction of information from a recording medium is accomplished by scanning the information pit row of the information track with a light beam spot having such a degree of predetermined power that recording is not effected on the medium, and detecting light reflected from or transmitted through the medium.
A single-light-source system and a plural-light-source system have been proposed as such information recording/reproducing systems. The typical construction of the single-light-source system is shown in FIG. 1 of the accompanying drawings.
In the apparatus of FIG. 1, a light beam emitted from a semiconductor laser 101 is made into a parallel light beam by a collimator lens 102, and this parallel light beam is divided into a plurality of light beams by a diffraction grating 103, and the plurality of light beams are condensed on an optical card 107 through a polarizing beam splitter 104, a quarter wavelength plate 105 and an objective lens 106. The reflected light from the optical card 107 passes through the objective lens 106, the quarter wavelength plate 105, the polarizing beam splitter 104 and a toric lens 108 to a photodetector 109. At this time, recording, reproduction and auto focusing control (hereinafter referred to as AF) are effected by the use of 0-order diffracted light of the light beams divided by the diffraction grating 103, and auto tracking control (hereinafter referred to as AT) is effected by the use of .+-.1st-order diffracted light. The so-called astigmatism method as described, for example, in U.S. Pat. No. 4,023,033 can be used for AF. Also, the so-called three-beam method as described, for example, in U.S. Pat. No. 3,876,842 is used for AT.
FIG. 2A of the accompanying drawings is a schematic plan view of an optical card. The optical card 107 has a number of information recording and reproducing tracks arranged parallel thereon, and some of the tracks are designated by T1, T2 and T3. These tracks are partitioned by tracking tracks tt1-tt4. The tracking tracks tt1-tt4 are formed by regions or grooves differing in reflectance from the tracks T1-T3, and are used as guides for obtaining a tracking signal. FIG. 2A shows an example in which information is recorded on or reproduced from the track T3. In this example, 0-order diffracted light 110 for recording, reproduction and AF is applied onto the track T3 and .+-.1st-order diffracted light 111 and 112 for AT are applied to the tracking tracks tt3 and tt4. A tracking signal which will be described later is provided by reflected light from the diffracted light 111 and 112 of the tracking tracks tt3 and tt4, and the 0-order diffracted light 110 properly scans the track T3. The diffracted lights 110 and 111 scan the optical card from left to right as viewed in FIG. 2A by a mechanism, not shown, while keeping the same positional relation. This scanning system includes a system for moving the optical system and a system for moving the optical card. In any case, the optical system and the optical card effect relative reciprocal movement and therefore, portions which are not constant in velocity are created at the opposite ends of the optical card.
This state is shown in FIG. 2B of the accompanying drawings.
The horizontal axis of FIG. 2B represents the left to right direction of the optical card, and the vertical axis represents the scanning velocity. Usually, the constant speed region in the central portion of the optical card 107 is used as a recording region.
FIG. 3 of the accompanying drawings is an enlarged view of portions of the diffracted light 110-112 of FIG. 2A. The 0-order diffracted light 110 for recording, reproduction and AF is situated in the middle between the .+-.1st-order diffracted light 111 and 112 for AT, and scans the center of the track T3. Hatched portions 113a, 113b and 113c indicate examples of the recording by the 0-order diffracted light 110 and are generally called pits. The pits 113a, 113b and 113c differ in reflectance from the surroundings thereof and therefore, when they are scanned by the light spot 110 of weak intensity after recording, the reflected light of the 0-order diffracted light 110 is modulated by the pits 113a, 113b and 113c, whereby there is obtained a reproduction signal.
FIG. 4 of the accompanying drawings shows the details of the photodetector 109 shown in FIG. 1 and a signal processing circuit. The photodetector 109 comprises four-division optical sensors 114 and optical sensors 115, 116, i.e., a total six optical sensors. Also, light spots 110a, 111a and 112a represent the reflected light of the diffracted lights 110, 111 and 112, respectively, in FIGS. 2A and 3. The light spot 110a is condensed on the four-division optical sensors 114, and the light spots 111a and 112a are condensed on the optical sensors 115 and 116, respectively. The sensor outputs of the four-division sensors 114 in the diagonal directions thereof are summed by addition circuits 117 and 118, respectively. The outputs of the addition circuits 117 and 118 are summed by an addition circuit 121 and provide an information reproducing signal RF. That is, the signal corresponds to all of the light spot 110a condensed on the four-division optical sensors 114. Also, the outputs of the addition circuits 117 and 118 are subtracted by a differential circuit 120 and provide a focusing control signal Af. That is, the signal Af is the difference between the sums of the four-division optical sensors 114 in the diagonal directions thereof. This astigmatism system is described in detail in the aforementioned U.S. Pat. No. 4,023,033. The outputs of the optical sensors 115 and 116 are subtracted by a differential circuit 119 and provide a tracking control signal At. Control is usually effected so that this signal At may be zero.
Designated by 111a and 112a are the reflected lights of the .+-.1st-order diffracted lights 111 and 112 in FIG. 3 as previously described. If the diffracted light 111 and 112 overlap with the tracking tracks tt3 and tt4, respectively, at the same rate, the light spots 111a and 112a will be of the same quantity of light. Accordingly, if control is effected so that the tracking control signal At may be zero, the 0-order diffracted light 110 will be situated the middle between the tracking tracks tt3 and tt4.
Turning back to FIG. 3, if the scanning loci of the 0-order diffracted light 110 during recording and during reproduction differ from each other, that is, if tracking deviates, the contrast of the signal RF and the time width of the pit portion may sometimes fluctuate and the reproduction of information may become impossible. Such a condition occurs due to the, vibration of the apparatus or dust on or a flaw on the optical card 107. It also occurs due to the residual when recording and reproduction are effected by discrete apparatuses. Particularly, in the case of the single-light-source system, recording and reproduction are effected by light spots of the same size and therefore, information cannot sometimes be reproduced even if the deviation between the tracking for recording and the tracking for reproduction is slight. Thus, the single-light-source system can be the to be small in the margin of tracking. Also, the diffracted light 110, 111 and 112 differ greatly in power between the pit recording period and the non-recording period and the light spots 110a, 111a and 112a likewise vary, and this has led to the problem of affecting AF control and AT control.